char ranges:

a.cpp

/// Is \p c an end-of-line character?
///
/// On Linux `\n` and on Windows `\r\n`.
template <typename Char>
constexpr bool is_eol(Char c, Char n = Char{}) noexcept {
  return c == '\n' || (c == '\r' && n == '\n');
}

/// Remove EOL characters from the \p line.
template <typename CharStr,
          CONCEPT_REQUIRES_(!std::is_rvalue_reference<CharStr&&>())>
constexpr auto strip_eol(CharStr&& chars) noexcept {
  auto eol_it = ranges::adjacent_find(chars, [](auto&& c, auto&& n) { return is_eol(c, n); });
  return ranges::make_iterator_range(ranges::begin(chars), eol_it);
}

{  // tests:
    test::check_equal(strip_eol("  a b \n"), view::c_str("  a b "));
    test::check_equal(strip_eol("a  b  c \r\n"), view::c_str("a  b  c "));

    test::check_equal(strip_eol("  c d\n"), view::c_str("  c d"));
    test::check_equal(strip_eol("c  d  e\r\n"), view::c_str("c  d  e"));

    test::check_equal(strip_eol("  d e "), view::c_str("  d e "));
}